JSON Web Token

JSON Web Token (JWT) is built on top of JSON Web Signature or JSON Web Encryption and includes specific payload claims. These claims are required to be in JSON format and follow a predefined set of fields.


Do you know that JSON Web Token (JWT) is not a part of JOSE. Instead, it was created by the OAuth working group.

Encode token

encode() is the method for creating a JSON Web Token string. It encodes the payload with the given alg in header:

from joserfc import jwt
from joserfc.jwk import OctKey

header = {"alg": "HS256"}
claims = {"iss": "https://authlib.org"}
key = OctKey.import_key("secret")
text = jwt.encode(header, claims, key)

The returned value of text in above example is:


Line breaks for display only.

Decode token

decode() is the method to translate a JSON Web Token string into a token object which contains .header and .claims properties:

# reuse the text and key in above example
token = jwt.decode(text, key)
# token.header = {'alg': 'HS256', 'typ': 'JWT'}
# token.claims = {"iss": "https://authlib.org"}

Validate claims

The jwt.decode method will only verify if the payload is a JSON base64 string.

You can define claims requests JWTClaimsRegistry for validating the decoded claims. The JWTClaimsRegistry accepts each claim as an Individual Claims Requests JSON object.

from joserfc.jwt import JWTClaimsRegistry

claims_requests = JWTClaimsRegistry(
    iss={"essential": True, "value": "https://authlib.org"},

# usually you will use the claims registry after ``.decode``

The Individual Claims Requests JSON object contains:


OPTIONAL. Indicates whether the Claim being requested is an Essential Claim. If the value is true, this indicates that the Claim is an Essential Claim.


OPTIONAL. Requests that the Claim be returned with a particular value.


OPTIONAL. Requests that the Claim be returned with one of a set of values, with the values appearing in order of preference.

And we added one more field:


OPTIONAL. Allow essential claims to be an empty string.

Missing essential claims

claims_requests = JWTClaimsRegistry(aud={"essential": True})

# this will raise MissingClaimError
claims = {"iss": "https://authlib.org"}

# this will raise MissingClaimError
claims = {"iss": ""}

Allow empty essential claims

claims_requests = JWTClaimsRegistry(aud={"essential": True, "allow_blank": True})

# this will NOT raise MissingClaimError
claims = {"iss": ""}

Invalid claims values

claims = {"iss": "https://authlib.org"}
claims_requests = JWTClaimsRegistry(iss={"value": "https://jose.authlib.org"})

claims_requests.validate(claims)  # this will raise InvalidClaimError

Default validators

The JWTClaimsRegistry has built-in validators for timing related fields:

  • exp: expiration time

  • nbf: not before

  • iat: issued at


JWT is built on top of JWS and JWE, all of the above examples are in JWS. By default jwt.encode and jwt.decode work for JWS. To use JWE, you need to specify a registry parameter with JWERegistry. Here is an example of JWE:

from joserfc import jwt, jwe
from joserfc.jwk import OctKey

header = {"alg": "A128KW", "enc": "A128GCM"}
claims = {"iss": "https://authlib.org"}
key = OctKey.generate_key(128)  # the algorithm requires key of 128 bit size
registry = jwe.JWERegistry()  # YOU MUST USE A JWERegistry
jwt.encode(header, claims, key, registry=registry)

The JWE formatted result contains 5 parts, while JWS only contains 3 parts, a JWE example would be something like this (line breaks for display only):


Another difference is the key used for encode and decode.

For JSON Web Signature, a private key is used for encode, and a public key is used for decode. The encode method will use a private key to sign, and the decode method will use a public key to verify.

For JSON Web Encryption, it is the contrary, a public key is used for encode, and a private key is used for decode. The encode method will use a public key to encrypt, and the decode method will use a private key to decrypt.

The key parameter

In the above example, we’re using OctKey only for simplicity. There are other types of keys in JSON Web Key.

Key types

Each algorithm (alg in header) requires a certain type of key. For example:

  • HS256 requires OctKey

  • RS256 requires RSAKey

  • ES256 requires ECKey or OKPKey

You can find the correct key type for each algorithm at:

Here is an example of a JWT with “alg” of RS256 in JWS type:

from joserfc import jwt
from joserfc.jwk import RSAKey

header = {"alg": "RS256"}
claims = {"iss": "https://authlib.org"}
with open("your-private-rsa-key.pem") as f:
    key = RSAKey.import_key(f.read())

# "RS256" is a recommended algorithm, no need to pass a custom ``registry``
text = jwt.encode(header, claims, key)

# ``.encode`` for JWS type use a public key, if using a private key,
# it will automatically extract the public key from private key
jwt.decode(text, key)

In production, jwt.encode is usually used by the client side, a client normally does not have the access to private keys. The server provider would usually expose the public keys in JWK Set.

Use key set

You can also pass a JWK Set to the key parameter of encode() and decode() methods.

import json
from joserfc.jwk import KeySet
from joserfc import jwt

with open("your-private-jwks.json") as f:
    data = json.load(f)
    key_set = KeySet.import_key_set(data)

header = {"alg": "RS256", "kid": "1"}
claims = {"iss": "https://authlib.org"}
jwt.encode(header, claims, key_set)

The methods will find the correct key according to the kid you specified. If there is no kid in header, it will pick on randomly and add the kid of the key into header.

A client would usually get the public key set from a public URL, normally the decode code would be something like:

import requests
from joserfc import jwt
from joserfc.jwk import KeySet

resp = requests.get("https://example.com/.well_known/jwks.json")
key_set = KeySet.import_key_set(resp.json())

def parse_token(token_string: str):
    jwt.decode(token_string, key_set)

Callable key

It is also possible to assign a callable function as the key:

import json
from joserfc import jwk

def load_key(obj):
    headers = obj.headers()
    alg = headers["alg"]
    key_path = f"my-{alg}-key.json"
    with open(key_path) as f:
        data = json.load(f)
        key = jwk.import_key(data["kty"], data)
    return key

# jwt.encode(header, claims, load_key)

Algorithms & Registry

The encode() and decode() accept an algorithms parameter for specifying the allowed algorithms. By default, it only allows your to use recommended algorithms.

You can find out the recommended algorithms at:

For instance, HS386 is not a recommended algorithm, and you want to use this algorithm:

>>> from joserfc import jwt, jwk
>>> header = {"alg": "HS384"}
>>> claims = {"iss": "https://authlib.org"}
>>> key = jwk.OctKey.import_key("secret")
>>> jwt.encode(header, claims, key, algorithms=["HS384"])

If not specifying the algorithms parameter, the encode method will raise an error.